In wireless communication systems, antenna diversity plays an important role in increasing the system link robustness. OFDM is used as a modulation technique for transmitting digital data using radio frequency signals (RF). In OFDM, a radio signal is divided into multiple sub-signals that are transmitted simultaneously at different frequencies to a receiver. Each sub-signal travels within its own unique frequency range (sub-channel), which is modulated by the data. OFDM distributes the data over multiple channels, spaced apart at different frequencies. The receiver needs to first detect the start of the packet and the OFDM symbol boundary for the FFT operation. Improper timing detection results in missing packet or inter-symbol interference (ISI). The receiver also needs to estimate the carrier frequency offset (CFO) and compensate it using signal processing techniques. Large CFO reduces carrier orthogonality, and introduces inter-carrier interference which degrades the system performance dramatically.
OFDM has been selected as the basis for the high speed wireless local area network (WLAN) standards by the 802.11a standardization group, and is also being considered as the basis for the high throughput WLAN 802.11n. As shown in FIG. 1, the packet preamble specified by the IEEE 802.11a standard consists of ten identical short training symbols (each containing 16 data samples) and two identical long training symbols (each containing 64 data samples). The short preamble (including the short training symbols) is used for timing detection and coarse frequency synchronization.
FIG. 2 shows a block diagram of a conventional correlation system for timing detection and coarse carrier frequency offset (CFO) estimation. The correlation system attempts to find the maximum/peak of the timing metric M(d) given by
      M    ⁡          (      d      )        =                                      P          ⁡                      (            d            )                                      2                      R        2            ⁡              (        d        )            
where d is a time index and
            P      ⁡              (        d        )              =                            ∑                      m            =            0                                L            -            1                          ⁢                                            r              ⁡                              (                                  d                  +                  m                                )                                      ·                                          r                *                            ⁡                              (                                  d                  +                  m                  +                  L                                )                                              ⁢                                          ⁢          and          ⁢                                          ⁢                      R            ⁡                          (              d              )                                          =                        ∑                      m            =            0                                L            -            1                          ⁢                                                        r              ⁡                              (                                  d                  +                  m                                )                                                          2                      ,
such that L=16 for the short preamble. Here, r(d) is the received time domain signal; m is a dummy variable for the summation; P(d) is the correlation result of the received signal r(d) with its delayed version r(d+L); R(d) is the auto-correlation result of the received signal r(d) itself; and L=16 is the number of data samples defined by the 802.11a standard short preamble.
The timing metric M(d) reaches a plateau which leads to some uncertainty as to the start of a packet frame. In addition, because L=16 is the same as the cyclic prefix in the OFDM symbol, peak detection of the timing metric sometimes provides wrong frame boundary. Other methods attempt to reduce the ambiguity due to the timing metric plateau. However, the false peak caused by the cyclic prefix (CP) still exists.
There is, therefore, a need for a new robust and accurate algorithm for timing detection and CFO estimation using the short preamble.